If you have said this about Apple/Google, I would have said ... maaaaaayyybeeeeee ...

But in this case, how exactly does Meta prevent people from India downloading and using another messaging app?

Nothing. It's a monopoly in a market that is heavily network dependent. Without interoperability, it's infeasible for a new option to become viable. Though I'm sure you know this already and just believe that monopolies are alright for consumers.

What would you believe is a solution?

WhatsApp is the core of Indian society. It's used by the government, all businesses, and everyone with a smartphone. It's used for payments, customer service, scheduling doctor's appointments, etc. You literally cannot replace it with something else anymore.